Add docs for missing variable substitution fields
Add docs for missing variable substitution fields

 This is documenting existing behavior - added command, args, script fields for containers (steps and sidecar).

See https://github.com/tektoncd/pipeline/blob/1f5980f8c8a05b106687cfa3e5b3193c213cb66e/pkg/apis/pipeline/v1beta1/container_replacements.go#L25 for implementation.
tektoncd/pipeline,tektoncd/pipeline